The present invention concerns additives for inhibiting or preventing the corrosion of metals in construction materials which comprise an inorganic binder, like cement, lime, hydraulic lime and gypsum, and optionally further constituents which are usually present in construction materials, like aggregates, materials which contain silicon dioxide and optionally further additives which improve the properties of the construction materials during their preparation and/or later on, when the construction materials are in use.
In many fields of the application, metals, like metal reinforcements, are embedded into construction materials. It furthermore been known since for many decades that inorganic binders which have basic properties, like cement and especially Portland cement, have the ability of protecting iron containing metals from the development of rust. Because of said rust protective effect of the inorganic binder, it was possible to use steel reinforced concretes in many fields of application. In said construction materials the steel reinforcements are embedded in the concrete and through the cement constituent of said concrete there is provided a durable corrosion resistance of the metal armings without applying onto it any protective coatings.
During the hydration of the cement, there is produced calcium hydroxide which raises a pH-value in the interior parts of the construction material to 12 or to a still higher value, and the corrosion protective activity of the cement is believed to be caused through said high pH-value.
In the corresponding construction materials during the use, however, by-and-by the carbon dioxide which is present in the environment converts the calcium hydroxide into calcium carbonate, and through said carbonate formation the pH-value in the construction material is lowered drastically. When the pH-value arrives at a value of 9 or lower, the corrosion of the steel arming embedded in the concrete starts and is proceeding by-and-by.
If during the use any cracks are formed in the construction material or if it comes into contact with chlorides then the corrosion of the embedded metal is accelerated. The contact of construction materials with chlorides or other metal corroding constituents can be caused through aggregates which contain any of said materials as impurities, through de-icing salts, through atmospheric pollution or through seawater.
In the prior art there, however, are not disclosed any additives which on one hand have the ability of preventing the corrosion of metals in construction materials during very long periods of time and with good reliability, and which on the other hand do not have a disadvantageous influence on any properties of the construction materials. Said problems are still more severe when the construction material is submitted to critical environmental influences, like e.g. brought into contact with chlorides or other corrosive agents.